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Bulk Transport
∗
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 3.0†
Abstract
By the end of this section, you will be able to:
• Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis
• Understand the process of exocytosis
In addition to moving small ions and molecules through the membrane, cells also need to remove and
take in larger molecules and particles (see Table 1 for examples). Some cells are even capable of engulng
entire unicellular microorganisms.
You might have correctly hypothesized that the uptake and release of
large particles by the cell requires energy. A large particle, however, cannot pass through the membrane,
even with energy supplied by the cell.
1 Endocytosis
Endocytosis
is a type of active transport that moves particles, such as large molecules, parts of cells,
and even whole cells, into a cell.
There are dierent variations of endocytosis, but all share a common
characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle.
The pocket pinches o, resulting in the particle being contained in a newly created intracellular vesicle
formed from the plasma membrane.
1.1 Phagocytosis
Phagocytosis (the condition of cell eating) is the process by which large particles, such as cells or relatively
large particles, are taken in by a cell. For example, when microorganisms invade the human body, a type of
white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulng
the microorganism, which is then destroyed by the neutrophil (Figure 1).
∗ Version
1.7: Apr 26, 2013 3:39 pm -0500
† http://creativecommons.org/licenses/by/3.0/
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Figure 1: In phagocytosis, the cell membrane surrounds the particle and engulfs it. (credit: Mariana
Ruiz Villareal)
In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes
coated with a protein called
clathrin,
which stabilizes this section of the membrane. The coated portion
of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing
it.
Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the
membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly formed
compartment (endosome). When accessible nutrients from the degradation of the vesicular contents have
been extracted, the newly formed endosome merges with the plasma membrane and releases its contents into
the extracellular uid. The endosomal membrane again becomes part of the plasma membrane.
1.2 Pinocytosis
A variation of endocytosis is called
pinocytosis.
This literally means cell drinking and was named at
a time when the assumption was that the cell was purposefully taking in extracellular uid.
In reality,
this is a process that takes in molecules, including water, which the cell needs from the extracellular uid.
Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge
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with a lysosome (Figure 2).
Figure 2: In pinocytosis, the cell membrane invaginates, surrounds a small volume of uid, and pinches
o. (credit: Mariana Ruiz Villareal)
A variation of pinocytosis is called
potocytosis.
This process uses a coating protein, called
caveolin,
on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin. The cavities
in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to
caveolin. The vacuoles or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis.
Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell
for their release on the other side of the cell, a process called transcytosis.
1.3 Receptor-mediated Endocytosis
A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specic
binding anity for certain substances (Figure 3).
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Figure 3: In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type
of substance that binds to the receptor on the external surface of the cell membrane. (credit: modication
of work by Mariana Ruiz Villareal)
In
receptor-mediated endocytosis,
as in phagocytosis, clathrin is attached to the cytoplasmic side
of the plasma membrane. If uptake of a compound is dependent on receptor-mediated endocytosis and the
process is ineective, the material will not be removed from the tissue uids or blood. Instead, it will stay
in those uids and increase in concentration. Some human diseases are caused by the failure of receptormediated endocytosis.
For example, the form of cholesterol termed low-density lipoprotein or LDL (also
referred to as bad cholesterol) is removed from the blood by receptor-mediated endocytosis. In the human
genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely.
People
with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear
LDL particles from their blood.
Although receptor-mediated endocytosis is designed to bring specic substances that are normally found
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in the extracellular uid into the cell, other substances may gain entry into the cell at the same site. Flu
viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and
gain entry into cells.
See receptor-mediated endocytosis in action, and click on dierent
:
parts
1
for a focused animation.
2 Exocytosis
The reverse process of moving material into a cell is the process of exocytosis.
Exocytosis is the opposite
of the processes discussed above in that its purpose is to expel material from the cell into the extracellular
uid. Waste material is enveloped in a membrane and fuses with the interior of the plasma membrane. This
fusion opens the membranous envelope on the exterior of the cell, and the waste material is expelled into
the extracellular space (Figure 4).
Other examples of cells releasing molecules via exocytosis include the
secretion of proteins of the extracellular matrix and secretion of neurotransmitters into the synaptic cleft by
synaptic vesicles.
1 http://openstaxcollege.org/l/endocytosis
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Figure 4: In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents
are then released to the exterior of the cell. (credit: modication of work by Mariana Ruiz Villareal)
Methods of Transport, Energy Requirements, and Types of Material Transported
Transport Method
Active/Passive
Material Transported
Diusion
Passive
Small-molecular weight material
Osmosis
Passive
Water
Facilitated transport/diusion
Passive
Sodium, potassium, calcium, glucose
continued on next page
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Primary active transport
Active
Sodium, potassium, calcium
Secondary active transport
Active
Amino acids, lactose
Phagocytosis
Active
Large
macromolecules,
whole
cells, or cellular structures
Pinocytosis and potocytosis
Active
Small molecules (liquids/water)
Receptor-mediated endocytosis
Active
Large
quantities
of
macro-
molecules
Table 1
3 Section Summary
Active transport methods require the direct use of ATP to fuel the transport.
Large particles, such as
macromolecules, parts of cells, or whole cells, can be engulfed by other cells in a process called phagocytosis.
In phagocytosis, a portion of the membrane invaginates and ows around the particle, eventually pinching o
and leaving the particle entirely enclosed by an envelope of plasma membrane. Vesicle contents are broken
down by the cell, with the particles either used as food or dispatched. Pinocytosis is a similar process on a
smaller scale. The plasma membrane invaginates and pinches o, producing a small envelope of uid from
outside the cell. Pinocytosis imports substances that the cell needs from the extracellular uid. The cell
expels waste in a similar but reverse manner: it pushes a membranous vacuole to the plasma membrane,
allowing the vacuole to fuse with the membrane and incorporate itself into the membrane structure, releasing
its contents to the exterior.
4 Review Questions
Exercise 1
(Solution on p. 9.)
What happens to the membrane of a vesicle after exocytosis?
a. It leaves the cell.
b. It is disassembled by the cell.
c. It fuses with and becomes part of the plasma membrane.
d. It is used again in another exocytosis event.
Exercise 2
(Solution on p. 9.)
Which transport mechanism can bring whole cells into a cell?
a. pinocytosis
b. phagocytosis
c. facilitated transport
d. primary active transport
Exercise 3
(Solution on p. 9.)
In what important way does receptor-mediated endocytosis dier from phagocytosis?
a. It transports only small amounts of uid.
b. It does not involve the pinching o of membrane.
c. It brings in only a specically targeted substance.
d. It brings substances into the cell, while phagocytosis removes substances.
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5 Free Response
Exercise 4
(Solution on p. 9.)
Why is it important that there are dierent types of proteins in plasma membranes for the transport
of materials into and out of a cell?
Exercise 5
(Solution on p. 9.)
Why do ions have a dicult time getting through plasma membranes despite their small size?
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Solutions to Exercises in this Module
to Exercise (p. 7)
C
to Exercise (p. 7)
B
to Exercise (p. 7)
C
to Exercise (p. 8)
The proteins allow a cell to select what compound will be transported, meeting the needs of the cell and
not bringing in anything else.
to Exercise (p. 8)
Ions are charged, and consequently, they are hydrophilic and cannot associate with the lipid portion of the
membrane. Ions must be transported by carrier proteins or ion channels.
Glossary
Denition 1: caveolin
protein that coats the cytoplasmic side of the plasma membrane and participates in the process of
liquid update by potocytosis
Denition 2: clathrin
protein that coats the inward-facing surface of the plasma membrane and assists in the formation
of specialized structures, like coated pits, for phagocytosis
Denition 3: endocytosis
type of active transport that moves substances, including uids and particles, into a cell
Denition 4: exocytosis
process of passing bulk material out of a cell
Denition 5: pinocytosis
a variation of endocytosis that imports macromolecules that the cell needs from the extracellular
uid
Denition 6: potocytosis
variation of pinocytosis that uses a dierent coating protein (caveolin) on the cytoplasmic side of
the plasma membrane
Denition 7: receptor-mediated endocytosis
variation of endocytosis that involves the use of specic binding proteins in the plasma membrane
for specic molecules or particles, and clathrin-coated pits that become clathrin-coated vesicles
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